Law enforcement officers are visible through the gate into the Navy Yard on Sept. 16. / Susan Walsh, AP

by Laura Petrecca and Donna Leinwand Leger, USA TODAY

by Laura Petrecca and Donna Leinwand Leger, USA TODAY

WASHINGTON -- Witnesses to the D.C. Navy Yard shooting described panic, confusion and terror as they scrambled out of Building 197, the scene of a shooting that left at least 13 people dead, including the shooter, and several injured.

A man with a gun "raised and aimed at us," witness Terrie Durham said. The man fired and "hit high on the wall just as we were trying to leave." Moments before, she had been working at her desk.

Another witness, Todd Brundidge, said that as he and others were trying to flee, a shooter "aimed the gun and fired our way."

Patricia Ward, who works in a building nearby, had stopped by Building 197 to use the ATM and get breakfast at the cafeteria. She heard gunshots.

"I heard pow, pow, pow," she said. "Then for a few seconds it stopped. And then pow, pow, pow."

Altogether she heard seven shots. The gunman appeared to be shooting from the fourth floor into the atrium, she said.

"I just started running," she said.

James Killingswoth, a mason from Frederick, Md., was working on rebuilding a historic wall outside the Navy Yard about 100 yards from the entrance when he heard two gunshots. If there were any other shots, he said, they were drowned out by wailing sirens.

"Everything they had, Secret Service, federal police -- everyone came speeding down the street. I've never seen so many police in my life," he said. "Scary morning."

Omar Grant, a civilian who works in network support at the Navy Yard, said he was on the first floor when he heard shots. He counted five.

"It was unmistakable," Grant said.

Alarms sounded immediately and he and his colleagues evacuated, leaving behind cellphones, keys and other personal belongings. Grant escorted a blind colleague from the building.

"It was hectic," he said.

Don Andres, 24, who works as a legislative aide and legislative correspondent on Capitol Hill, was driving past the area and saw a "lot of confusion, a lot of emotion."

At least 12 people had been confirmed dead by 2 p.m., Mayor Vincent Gray said.

Matt Goetz of Arlington, Va., sent a few Twitter updates of his status while he was in lockdown at the Navy Yard's Admiral Gooding Center. He tweeted, "I am fine here at the Yard and safe. The Internet is terrible so I can't really send much out, but I am fine."

Other people used Twitter to send updates about their safety status and to say if they were still waiting to hear from loved ones.